Process and system for rapid analysis of the spectrum of a signal at one or several points of measuring

ABSTRACT

In electron beam measuring, it is often also necessary to measure the frequency range in addition to the measuring time range. In order to do this, according to the invented process, the output signal of the local oscillator of a spectrum analyzer, as is known from conventional high-frequency measuring, undergoes a first frequency conversion and subsequently is utilized for modulating the primary beam. Based on the potential contrast as a multiplicative interaction and the modulated primary beam, the under circumstances very high-frequency signal to be analyzed is transformed to an easily detected low intermediate frequency. Subsequently this intermediate frequency signal is transferred into an input frequency plane of the spectrum analyzer by a second frequency conversion. Both the variable input selection frequency or the fixed intermediate frequency of the spectrum analyzer may be the frequency . The measured result appears in the usual manner on the display of the spectrum analyzer.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a method and apparatus for performing arapid analysis of the spectrum of a signal at one or several points ofmeasurement, and for determining the spatial distribution of individualspectral lines.

Analysis of the mode of function of high-integrated circuits is usuallyperformed in computer-controlled test systems, in which errors can beidentified by analyzing the voltage level detected at the outputs of theexamined circuit dependent on the respectively fed bit pattern; however,such measurement points can be located only with great difficulty. Forthis reason, additional measuring must be conducted insidehigh-integrated circuits, particularly during the development phase.

Particle beam measuring processes, especially electron beam measuring,used in all fields of development and fabrication of micro-electroniccomponents, have proven particularly well suited for this purpose. Withthe aid of these measuring techniques, for example, the electricpotential distribution in integrated circuits can be imaged ("voltagecoding" and "logic state mapping") or the temporal potential course canbe determined at a single point of junction (wave-form measuring). Asurvey of the test procedures currently generally employed is given inthe publications "Electron Beam Testing" by E. Wolfgang("Microelectronic Engineering", issue 4, 1986, pages 77-106) and"Electron Beam Testing" by K. Ura and H. Fujicka ("Advances inElectronics and Electron Physics", volume 73, 1989, pages 233 -317).

An important object of these processes is to determine whether or not asignal of a specific frequency is at a particular conductor channel, andwhat the frequency spectrum of the signal is. Another object is todiscover which conductor channels carry a specific signal (and therewitha specific signal frequency). The frequency tracing and frequencymapping processes, which were developed for this purpose are describedin detail in the publication "Frequency Tracing and Mapping in Theoryand Praxis" by H. D. Burst and F. Fox ("Microelectronic Engineering"volume 2, 1984, pages 299-323). These processes are especially useful inexamining asynchronous circuits, for which other processes based onsampling techniques are unsuccessful due to insufficientsynchronization.

Unfortunately, in prior art frequency range methods of frequency tracingand mapping, a spectrum analysis can be conducted only quite slowly forreasons which will be explained later, and previous proposals toaccelerate the process based on the principle of velocity modulation(described in German patent applications 454 and DE 35 10 525) arerelatively complicated and expensive. Moreover, conducting measuring ofthis type differs considerably from conventional measuring in the fieldof high-frequency technology, which presents problems to a user who isnot particularly familiar with electron beam testing.

The object of the present invention is to provide a simple, low costmethod and apparatus for rapidly analyzing a spectrum of a signal andrepresenting the spatial distribution of the spectral lines.

This object is achieved by a process and apparatus in accordance withthe present invention in which the sample circuit is irradiated by aprimary beam which interacts with the sample in a manner dependent onthe quantity to be analyzed, and a secondary signal is derived which isindicative of said interaction. The output signal of the localoscillator of a spectrum analyzer, after undergoing a frequencyconversion, is used to modulate the primary beam. The frequency to beanalyzed, as contained in the secondary signal, is transformed to aneasily detected low intermediate frequency, and transferred into aninput frequency range of the spectrum analyzer. The measured result isdisplayed on an output CRT of the spectrum analyzer.

A principal advantage of the present invention lies in its ability tofacilitate rapid measurement, which is less taxing on the sample to beexamined. In contrast to prior art processes, rapid examination alsopermits continuous monitoring of a signal at a measuring point.Moreover, the measuring process and the collateral system are the sameas in conventional high-frequency measuring, although the interpretationof the results differ somewhat. In this way, the user requires notraining time.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the fundamental principle of the frequency range methods;

FIG. 2 depicts a system for conducting the conventional frequencytracing process;

FIG. 3 depicts a system for conducting the conventional frequencymapping process;

FIG. 4 depicts a system for performing the process according to theinvention;

FIG. 5 depicts a modification of the system of FIG. 4 for measuring thespatial distribution of individual spectral lines; and

FIG. 6 an example of a measuring result such as can be obtained with thesystem of FIG. 4 or 5.

The same designations in the figures identify the same signals and thesame parts of the circuit throughout the drawing.

DETAILED DESCRIPTION OF THE DRAWINGS

Although the following preferred embodiments refer to an electron beammeasuring device, the present invention is to be understood in such amanner that ions or other particles may be utilized instead ofelectrons, both as primary particles and secondary particles. Just asradiation, such as laser or X-ray radiation, may also be used instead ofprimary, respectively secondary particles.

As illustrated in FIG. 1, in the electron beam measuring processes, afinely focused primary electron beam is directed at the measuring pointof the integrated circuit to be examined. The primary electronsimpinging thereupon release secondary electrons from the surface of thesample, which are influenced by the electric potential of the surface ofthe sample. This influence manifests itself in a secondary electronstream which is dependent on the potential, and in an energy shift ofthe secondary electrons which also is determined by the electricpotential at the measuring point and can be measured with the aid of anenergy spectrometer. This effect is called a potential contrast.

In principle, it should suffice to impinge primary electrons upon themeasuring point, to register the corresponding secondary electron streamand to guide the secondary electron signal to a conventional spectrumanalyzer for spectrum analysis. Indeed, this can be done, but only,however, if the signals to be examined have relatively low frequencies,because the detectors required for registering the secondary electronsusually only have a relatively small bandwidth of several MHz.Therefore, a scanning process is utilized in which a triggering signalcauses the temporal course of the signal to be scanned at the measuringpoint with short electron pulses in the same manner as with a samplingoscilloscope. In this manner, high time resolution can be achieved, evenwith time range processes.

In frequency range processes, "frequency tracing" and "frequencymapping", a different technique is employed in order to overcome thisdifficulty. The signal frequency to be examined which is usually quitehigh, is mixed down to a low intermediate frequency, which the secondaryelectron detector can easily transmit, before the detector limiting thebandwidth. The potential contrast is utilized as nonlinear interactionfor this mixing process. FIG. 1 depicts this principle. The potentialcontrast ensures that the secondary electron stream i_(SE) isproportional to the primary electron stream i_(PE), and that it isdependent on the signal u(t) at the measuring point. Thus, therelationship

    i.sub.SE(t) =i.sub.PE (t) * g(u(t))                        (1)

holds true, with g standing for the characteristic of the potentialcontrast, that is the relationship between the signal u(t) at themeasuring point and its influence on the secondary electron streami_(SE). In the first approximation this characteristic is consideredlinear, and thus the relationship (1) is treated as purelymultiplicative. If the signal u(t) has a signal frequency f_(S) in itsspectrum, this signal frequency can be mixed down to the lowintermediate frequency f_(ZF) by modulating the primary electron beam PEand thereby the primary electron stream i_(PE) (t) with a frequency off_(B), which is minimally offset, namely by f_(ZF), against the signalfrequency f_(S) to be measured. Precisely said, the mixing condition

    |n f.sub.B -m f.sub.S |=f.sub.ZF         (2)

has to be met. The following reflections are based on a fundamental wavemixing, thus n=m=1, without the intention of limiting the scope orspirit of the present invention. Harmonic wave mixing ensuesanalogously, only that in the subsequent relationships f_(b) has to bereplaced by nf_(a), respectively f_(a) by mf_(S).

FIG. 2 shows how this fundamental principle is realized in frequencytracing processes. The spatial distribution of individual spectral linescan be determined by frequency tracing processes, i.e., all themeasuring points displaying a signal of a specific frequency in thesignal spectrum, are imaged. FIG. 2 depicts schematically a system, withwhich the frequency tracing process can be realized. The principalfeature of this system, as well as of the system described hereinafter,is, by way of illustration, an electron beam measuring device such as isdisclosed in U.S. Pat. Nos. 4,220,853 and 4,223,220, and a rasterelectron microscope. A finely focused primary electron beam PE isgenerated in the electron-optical column of such an electron beammeasuring device. In addition to a multiplicity of shutters and electricor magnetic lens systems for beam shaping, beam deflection and beamfocusing, which are not depicted in the subsequent figures for reasonsof clarity, this electron-optical column is provided with an electronsource ES, which generates the primary beam PE.

The function of the electron source ES is to generate the primaryelectron beam modulated with a frequency of fB. In FIG. 2, it isessentially composed of an electron gun EG consisting of a cathode,which generates the primary electrons by thermal emission, and anode andWehneltelectrode, and a beam modulating or scanning system BBS. Thecontinuous stream of primary electrons supplied by the cathode ismodulated in its intensity with the aid of the beam modulation systemBBS. In this way, a primary electron stream having a frequency of f_(b)can be generated. Possible embodiments of th electron gun EG and beammodulation system BBS are, by way of illustration, described in thepublication "Electron beam chopping systems in the SEM" by E. Menzel andE. Kubalek (Scanning Electron Microscopy, SEM Inc., AMF O'Hare, 1979/I,pages 305-317). A well suited beam modulation system is, by way ofillustration, disclosed in U.S. Pat. No. 4,169,229.

In order to generate the modulated primary electron stream, a beammodulation generator BBG is connected to the modulation input MI of theelectron source ES. The beam modulation generator BBG triggers the beammodulation system BBS with a modulation signal MS. In this embodiment,the modulation signal MS is composed of square pulses occurring with aconstant recurrence frequency of f_(B). Other possible ways oftriggering the beam modulation system with signals of various shape arealso described in the cited publication by E. Menzel and E. Kubalek andcan, in principle, also be utilized. Every square pulse of themodulation signal MS keys the primary electron beam BE and generates aprimary electron pulse in this manner. The width of the square pulsedetermines the duration of the primary electron pulse. In contrast tothe time range methods, the beam modulation generator BBS is notsynchronized with the rest of the measuring system.

The primary electron pulses generated in this manner are focused by thelens system (not shown) onto the sample IC, such as an integratedcircuit. There the impinging primary electrons PE release secondaryelectrons SE, which are registered by detector DT, and are convertedinto light pulses. These light pulses are then guided via a light guideto the photomultiplier PM usually located outside the sample chamber ofthe electron beam measuring device, which turns the light pulses into anelectric signal for further evaluation as a secondary (in this caseelectric) signal SS, if necessary following further amplification in apre-amplifier PA connected after the photomultiplier PM.

For the reproduction of processes of interest, e.q., an error, thesample IC to be examined is operated cyclically. Therefore, the sampleIC is supplied with voltages and, if need be, input signals forstimulation by a triggering means ICA. In particular, a function testermay serve as the triggering means ICA.

If the signal frequency f_(S) occurs at the measuring point, a portionof the signal appears in the secondary electron stream at theintermediate frequency of f_(ZF), This portion of the signal is filteredout with the aid of a bandpass filter BP, which is fixedly synchronizedwith the intermediate frequency f_(ZF), demodulated in an envelopemodulator AMD and then fed (if need be following a comparison with athreshold value TH in a comparator CM) to the z-input of the screen CRTof the electron measuring device thus brilliance modulating the screenwriting beam. Scanning the primary electron beam PE subsequently overthe sample--by means of appropriately triggering the deflection coils ofthe electron beam measuring device--and, as is customary, also movingthe writing beam of the picture tube CRT synchronously with it, resultsin a brilliance distribution, which corresponds to the spatialdistribution of the signal frequency f_(S), selected by the specialselection of the modulation frequency f_(B) via mixing condition (2), onthe scanned area of the sample.

If the spectrum of a signal at a measuring point is to be determined, itsuffices to sweep through the modulation frequency over the frequencyrange of interest. This occurs in the so-called frequency mappingprocess. A system for performing this process in shown in FIG. 3, whichdiffers only slightly from the system of FIG. 2. In the frequencymapping arrangement of FIG. 3, the beam modulation generator BBG is avoltage controlled oscillator (VCO) supplied by a ramp generator SG witha control voltage, which determines the output frequency f_(B). Thiscontrol voltage is also applied simultaneously at the y-deflection ofthe writing beam of the picture tube, while the x-deflection continuesto be steered by the deflection generator RG, which also moves theprimary electron beam. In this process, the primary electron beam isdeflected only in one direction along one line. The modulation frequencyf_(B) is swept over the frequency range of interest by the ramp-shapedcontrol voltage. As the primary electron beam is moved at the same time,a spectrum analysis of the signals along one line of measuring pointsoccurs in this manner. The level of the spectrum lines is shown on thescreen CRT by the brilliance of the corresponding points.

In frequency tracing and frequency mapping processes, the beammodulation generator is usually a frequency synthesizer. Unfortunately,synthesizers have relatively long switching and oscillation times whenswitching from one frequency to the next, as is necessary in sweepingthe modulation frequency so that sweeping the modulation frequency f_(B)can occur only relatively slowly. In a certain sense, in frequencymapping processes this necessity is turned into a virtue in that asignal is analyzed not only at a single measuring point per frequencystep, but at an entire series of measuring points along a line. However,the reading precision when reading the measured values from the screenof the electron beam measuring device is relatively low.

These problems are avoided by the process according to the invention,which is also based on the fundamental principle of the other frequencyrange methods; that is, mixing the relatively high signal frequencyf_(S) to a low intermediate frequency f_(ZF), which is easy for thedetector to transmit. The fundamental idea of the process is to utilizea conventional spectrum analyzer familiar to those versed in the fieldof high frequency measuring, such as Tektronix mode 2756 P. (Thebuild-up and operation mode of such spectrum analyzers is familiar tothose versed in the art, and described e.q., in the company journals"Measuring with a Spectrum Analyzer" of Hewlett-Packard, and "Principlesof Spectrum Analysis with the Spectrum Analyzer FSA" of Rohde &Schwarz.) The spectrum analyzer is connected to an electron measuringdevice according to the invention. This arrangement is advantageous notonly because a spectrum analyzer has internally a rapid sweep generatorat its disposal, but also because the detection circuits of a spectrumanalyzer are usually oscillation-optimized and thus have particularlyshort reaction time.

Most spectrum analyzers work according to the superimposition principle.In contrast to the frequency range methods of electron beam measuring,however, the input signal is mixed to higher internal intermediatefrequency f' in order to avoid ambiguity due to image frequencies, andis limited in its bandwidth by a low-pass filter. In Tektronix model2756 P, this first intermediate frequency lies at 2072 MHz as long asoperation is in the fundamental band and only signals up to 1.8 GHz areto be analyzed. The local oscillator, which triggers input mixer of thespectrum analyzer, has to be varied in its frequency f_(LO) between 2072and 3872 MHz. This local oscillator frequency f_(LO) or one of thesubsequent steps of the analyzer is usually available at an output ofthe spectrum analyzer. This is particularly the case, if the spectrumanalyzer is prepared for connection to an external mixer. In this event,a local oscillator frequency can be tapped at the output to theconnection of the external mixer.

According to the invention, the local oscillator frequency f_(LO) isthen utilized to modulate the primary electron beam following afrequency conversion. The frequency conversion is necessary as the localoscillator frequency usually does not lie in the desired range due tothe upward mixing in the spectrum analyzer. In most cases it lies muchtoo high. If work is to be done in the fundamental band of the spectrumanalyzer, the local oscillator frequency has to be reduced by theinternal intermediate frequency f' minus the desired intermediatefrequency f_(ZF). Thus for the modulation frequency f_(B)

    f.sub.B =f.sub.LO -f'+f.sub.ZF                             (3)

holds true, the value of the intermediate frequency f_(ZF) being, inprinciple, random. It only has to be sufficiently low that it can betransmitted by the detector without difficulty, and should lie in arange in which as few as possible distorted signals and little noiseoccurs. In the conventionally employed secondary electron detectors, theuseful frequency range for the intermediate frequency f_(ZF) liesbetween 250 kHz and 3 MHz. Thus if a frequency conversion according torelationship (3) is carried out, the spectral portions, in particular,of the signal to be examined having the signal frequency

    f.sub.S =f.sub.LO -f'

and naturally also the respective image frequency

    f'.sub.x =f.sub.LO -f'+2*f.sub.ZF

are mixed by the potential contrast in the intermediate frequency rangef_(ZF). If the example of the Tektronix analyzer 2756 P is the basis andan intermediate frequency f_(ZF) =1 MHz is selected, the modulationfrequency varies between 1 MHz and 1801 MHz when the spectrum analyzersweeps over the fundamental band. Accordingly, during sweeping of theentire fundamental band, the spectral portions of the signal u(t) to beexamined are mixed between 0 MHz and 1800 MHz, and between 2 MHz and1802 MHZ, at the measuring point by the potential contrast in theconstant intermediate frequency range at f_(ZF).

The system according to the invention is depicted in FIG. 4 wherein thesample, sample triggering, primary beam source ES and detection systemDT, do not initially differ materially from the systems depicted FIGS. 2and 3, and the description of these components, given in the systems ofFIGS. 2 and 3, is also valid in the same manner for the inventedprocess. The manner of obtaining the modulation signal, however, differsfrom that in the prior art processes. The output signal LO of the localoscillator of the spectrum analyzer SPA having the frequency f_(LO) istransmitted to a first input of a frequency converter M1, preferably amixer. A signal having the frequency f'-f_(ZF) is transmitted to thesecond input of the frequency converter. In the system according to FIG.4, this signal is generated by a first signal generator G1, which ispermanently tuned to this frequency, and the output of which isconnected to the second input of the frequency converter. The signal ofthe frequency f'-f_(ZF) can also be obtained from an output frequency ofthe spectrum analyzer, preferably from the reference frequency, with theaid of a PLL circuit instead of with a fixedly tuned first signalgenerator G1. The design of PLL circuits of this type is familiar tothose versed in the art.

The output signal of the frequency converter M1, having a frequencywhich is equal to the difference of its two input frequencies, isutilized, if need be, to modulate the primary electron beam followingamplification in an amplifier connected thereafter. For this purpose,the output of frequency converter Ml is, if need be, connected via theamplifier connector therebetween, to an input of the beam modulationgenerator BBG, which makes a square-shaped modulation signal MS fortriggering the beam modulation system BBS from the sinusoidal outputsignal of the frequency converter M2. To do this, the generator shouldbe operated in the "external width mode". By way of illustration, theHewlett-Packard Type 8080 may be employed as the beam modulationgenerator. However, the beam modulation system BBS can be directlytriggered with the sinusoidal output signal of the frequency converterM1, and of the amplifier. In this event, the beam modulation generatorBBG may be dispensed with, and replaced by a passive "bias-T" in orderto superimpose a direct voltage for optimum focusing of the point ofoperation.

The spectral component of interest now lies after the detector DT andthe pre-amplifier PA in the intermediate frequency range thus atf_(ZF'). In order to facilitate the actual spectrum analysis by thespectrum analyzer SPA, it is necessary to "pretend to" the spectrumanalyzer that this signal component is its input signal. For thispurpose, the spectral component has to be transformed from theintermediate frequency range into an input frequency range of thespectrum analyzer. The input frequency range may be the actual, variableinput selection range or one of the usually fixed internal intermediateranges of the spectrum analyzer. The required frequency transformationonce again occurs with the aid of a frequency converter.

First of all, the case is described in which the spectral component isto be transformed in the input selection range. If the local oscillatorhas a frequency of f_(LO), the signal is just being analyzed at thefrequency f_(LO) -f' at the input of the spectrum analyzer SPA. This isthe input selection range, and as it is dependent on the localoscillator frequency f_(LO), it is not constant but variable. Therefore,the frequency of the spectral component of interest must be raised fromf_(ZF) to F_(LO) -f'. To do this, in FIG. 4, the secondary signal, SSand therewith the signal component of interest, is transmitted at f_(ZF)to a first input of a second frequency converter M2, which again may bea mixer. As can be seen from equation (3) above, the desired frequencytransformation may occur simply by applying the modulation frequencyf_(B) to the second input of the frequency converter M2, for the sumfrequency and the difference frequency of both its input signals appearat the output of a mixer, which can be utilized as the frequencyconverter. For this purpose, the second input of the second frequencyconverter M2 is connected to the output of the first frequency converterMl, if need be, via an amplifier V connected therebetween. The spectralcomponent of interest at the output of the second frequency converter isthen in the input selection range of the spectrum analyzer, so that theoutput of the frequency converter M2 may be simply connected to theselection input Il of the spectrum analyzer. This relationship ismaintained independent of the respective current frequency f_(LO) of thelocal oscillator, so that the spectrum analyzer always registers theamplitude of the current spectral components of the signal to beexamined.

FIG. 5 shows a modification of the invention, in which the spectralcomponent of interest is transformed from the intermediate frequencyrange at f_(ZF) into an internal intermediate frequency range of thespectrum analyzer SPA. It differs from th embodiment in FIG. 4 only inthe wiring of the second frequency converter M2, if f" is the fixedinternal intermediate frequency range, which is utilized as the inputfrequency range, the frequency f"-f_(ZF) or the frequency f"+f_(ZF) hasto be transmitted to the second input of the second frequency converterM2. A second signal generator G2 supplies this frequency, the output ofwhich is connected for this purpose to the second input of the secondfrequency converter M2. As an alternative, the frequency f"-f_(ZF), andf"+f_(ZF) is derived from the spectrum analyzer SPA by means of a PLLcircuit. The output of the frequency converter is connected to the inputI2 of the selected internal intermediate frequency range of the spectrumanalyzer SPA. Contrary to the arrangement of FIG. 4, which can be useduniversally, this modified system presumes that an input of this kindexists, which unfortunately is not true of all commercial devices.

Regardless of which of the two described embodiments is utilized, in theprocess according to the invention, the actual evaluation of thespectrum analysis is carried out by the spectrum analyzer SPA. Operationalso is as is customary in high-frequency measuring with the spectrumanalyzer. The measured result appears on the screen of the spectrumanalyzer SPA, not on the electron beam measuring device as in the priorart processes. The electron beam measuring device is actually used onlyfor positioning the primary electron beam onto the measuring point,which simplifies operation even for the layman, as special training isnot necessary. Only the interpretation of the obtained results differsslightly from the usual image. That is, due to the mixing condition (2)a double peak appears at the measuring point in the measured spectrumfor each spectral component in signal u(t), whereby (in the scale of thespectrum analyzer) the upper peak corresponds to the actual frequency ofthe measured spectral component and the lower peak lies exactly 2+f_(ZF)below it, as illustrates in FIG. 6. In the top diagram a) a spectrum ofthe signal at the measuring point is illustrated; in the lower diagramb) the spectrum measured with the invented process consists of two pairsof peaks, characterizing the two spectral lines of the measuring signal.

Operating exclusively via the spectrum analyzer SPA also permitsutilization of all extended operation types of a spectrum analyzer.Among these extended types of operation is, by way of illustration,automatic peak detection, tracking type of operation (in which theanalyzer automatically tracks the frequency drift of a spectral line)and the monitoring type of operation (in which the analyzer measures andindicates the temporal change in the spectrum).

The first frequency conversion (with the aid of the first frequencyconverter Ml) does not necessarily have to occur at the frequencyf-f_(UF) ; any frequency is possible as long as the second frequencyconversion occurs in an input frequency range of the spectrum analyzerSPA (which, by way of illustration, is automatically the case in theembodiment according to FIG. 5). In that event, a frequency conversioncan occur at a very high or also a very low frequency. In extreme cases,the frequency zero is even permissible; that is, the frequencyconversion may be completely dispensed with. The generator G1 is, ofcourse, superfluous in that case and the first frequency converter Mlmay be a simple connecting piece.

Another embodiment of the invented process also permits easy measurementof the spatial distribution of a spectral component. For this purpose,the medium frequency of the spectrum analyzer SPA is set at the signalfrequency of interest, the local distribution of which is to bedetermined, and the "zero-span" type of operation of the spectrumanalyzer is selected. The spectrum analyzer SPA then measures thetemporal change of this spectral line. Subsequently the demodulatedoutput signal of the last step of the spectrum analyzer SPA (the signalwhich causes the y-modulation of the spectrum analyzer picture tube,usually called a video signal) is transmitted to the z-input (brilliancemodulation) of the display CRT of the electron beam measuring device, ifneed be, following additional signal processing (e.g. level adjustmentor a comparison with a threshold value in order to suppress undesireddisturbances). This is shown with a broken line in FIGS. 4 and 5. If theprimary electron beam PE (as in the conventional frequency tracingprocess) scans the sample IC synchronously with the writing beam whichscans the screen CRT of the electron beam measuring device, the spatialdistribution of the spectral lines of interest appears on the screen asbrilliance distribution.

A further embodiment of the invented process yields a result likesimilar to conventional frequency mapping processes. As with theprevious embodiments, the video output signal of the spectrum analyzerSPA is connected to the z-input of the screen CRT of the electron beammeasuring device. However, the spectrum analyzer SPA is not operated inthe "zero span" but rather in the conventional sweep type operation. Asin frequency mapping processes, the primary electron beam PE ispermitted to scan only along one line, and the writing beam of thescreen CRT scans synchronously to it in one direction, by way ofillustration, in the y-direction. The scanning of the writing beam inthe perpendicular direction to it, that is, the x-direction, now has tooccur synchronously to the sweep of the spectrum analyzer SPA. For thispurpose, the sweep signal of the spectrum analyzer SPA (frequentlycalled "sweepscan"), and thus the ramp-shaped signal indicating the justanalyzed frequency, may be used in order to deflect the writing beam.Alternatively, in a reverse manner the x-deflection voltage of thescreen CRT may be transmitted to the spectrum analyzer SPA as anexternal sweep signal.

Hitherto, the subject hereof has been a spectrum analyzer, which in thiscontext is understood to mean a device which conducts a spectrumanalysis, and in addition sweeps through the frequency range ofinterest. It should be noted in this regard that a so-called networkanalyzer also falls within this definition. A device of this kindmeasures not only the amplitude spectrum, as does a conventionalspectrum analyzer, but also the phase spectrum. A network spectrum ofthis kind may therefore also be employed for the realization of theinvented process.

According to another embodiment of the invention, an extension of thepossible applications of a network analyzer is utilized, and the phasespectrum of signal u(t) is also measured at the measuring point. Forthis purpose, the sample triggering must be synchronized with thenetwork analyzer, for example via a common reference frequency.Furthermore, the phase relationships of the participating signals mustalso be maintained in the frequency conversions, which can be ensured byusing mixers as the frequency converters Ml and M2, and by makingcertain that the output signal of the first and, if need be, also thesecond signal generator G1 and G2 are synchronized with the networkanalyzer. This can be easily achieved by designing these generators asPLL circuits, which derive their output signal from a referencefrequency of the spectrum analyzer. In this manner, the amplitude andphase spectrum of signal u(t) at the measuring point can be measuredsimultaneously. With these two items of information, the temporal courseof the signal u(t) at the measuring point can be determinedmathematically by a Fourier reverse retransformation in that time range.

There are, of course other possible embodiments of the electron sourceES than the one shown in FIGS. 2-5. Thus, for example, instead of theheated cathode, which generates primary electrons PE by means of thermalemission, a field-emission cathode or a photo-cathode stimulated to emitelectrons by laser pulses can be utilized. Also, a semiconductor cathodemay be employed, in which case the intensity of the emission can veryeasily be controlled by varying the cathode current. An independent beammodulation system after the cathode can then be dispensed with, and themodulation signal MS controls the cathode current directly.

Naturally, other detectors can be used for deriving the secondary signalbesides the secondary electron detector described in connection withFIGS. 2-5. For example scintillation counters, Faraday cages orsemiconductor detectors may be used. A means for multiplying thesecondary electrons (such as, for example, a channel plate) may, ofcourse, always be connected before the detector itself. In principle,any detector which releases a measuring signal when secondary electronsimpinge thereupon may be used. In particular, an energy spectrometer Spcan be used to derive the secondary signal SS. This is indicated by acounterfield network in the FIGS. 2-5. Especially well suited for thispurpose is a counterfield spectrometer, such as disclosed, for examplein U.S. Pat. No. 4,292,419, which can be utilized to obtain in a mannerfamiliar to those versed in the art. A particularly easy possibility is,e.q., applying a constant voltage to the counterfield network.

The present invention is also not to be understood as limited toembodiment in which the primary beam PE is finely focused on themeasuring point. Rather within the scope of the present invention theprimary beam PE can also irradiate large areas of the sample, and thedefinition of the point of measurement occurs only through locallyresolved gaining of the secondary signal SS. For example, the surfacepotential may be measured with photo-electrons by means of the potentialcontrasts by irradiating the sample with light over its entire surface(the primary beam PE is therefore substantially widened), registeringthe generated photo-electrons SE separately with the aid of amulti-channel detector according to their point of origin andtransforming them into a secondary signal.

The mixing to the intermediate frequency f_(ZF) is based on a non-linearrelationship and the modulation of the primary electron beam. However,it can also be achieved by modulating the intensity of the secondaryelectrons or the secondary signal (SS) instead of the primary electrons,for example by modulating the energy threshold of an opposite-fieldspectrometer with the modulation frequency of fB. By taking into accountthe spectrometer characteristic, a sinusoidal modulation of thesecondary electron signal can even be obtained, thereby avoidingpossible difficulties with cross-modulation products, as they cansometimes occur in the described processes. However, the criticalfrequency is lower due to the higher capacities to be transferred andthe energy dispersion of the secondary electrons. Moreover, the field ofvision is limited by the spectrometer. Analogously, the photo-multipliercan also be operated together with a gate circuit or modulated in thevideo-signal path. In this event, the attainable bandwidth, however, isnarrower.

The present invention has hitherto been described by using the potentialcontrast effect in an electron beam measuring device. Its use, however,is by no means restricted thereto. Any other particles, such as forexample ions or any radiation, in particular light radiation can beutilized instead of primary and secondary electrons. Thus, a laser beamis used as the primary beam PE, it can cause the release ofphoto-electrons on the surface of the sample, which are then influencedby potential contrast of the electric fields on the surface of thesample, and can be detected, as previously described, as secondaryelectrons. The potential contrast can, however, also be replaced byother interactions, such as the influence of a magnetic field on thesecondary electrons SE generated by a primary electron beam PE.Utilization of this so-called "magnetic contrast" permits examination ofthe movement of magnetic domains in magnet bubble memories. In addition,the secondary signal SS need not come from a secondary particle stream,which is registered with the aid of a detector. Just as, by way ofillustration, the secondary signal may be derived directly from thesample e.q., by measuring the current induced by the primary beam PE inthe sample IC. An example of such a technique is EBIC (electron beaminduced current), which is well-known to those versed in the art.

Of course, the various modifications can be combined and employed inthat manner. If a laser beam is used as the primary beam PE and anintegrated circuit as the sample IC, the laser beam can generateelectron-hole pairs in the pn-junction of the sample IC and thereby freecharge carriers. This then becomes evident in the altered current inputof the sample. The magnitude of this change also depends on theswitching condition of the respective pn-junction. A change in theswitching conditions of a pn-junction can be easily determined,therefore, by measuring the supply current of the sample IC. The supplycurrent of the sample, and its deviation from the closed circuitcurrent, can in this case serve directly as the second signal SS, and aspecial detector therefore is unnecessary. A laser beam can also beutilized as the primary beam in order to measure the surface potentialof the sample IC. The interaction, which provides the multiplicativerelationship, is in this case constituted by electro-optical effects.For this purpose, an electro-optical crystal is placed on the surface ofthe sample. In order to obtain the second signal, the primary beam PEcan be aimed at the electro-optical crystal and the reflected lightguided via a polarizer and transmitted to a detector, for example, aphoto multiplier. If the electro-optical crystal rotates thepolarization range of the light in response to the surface potential ofthe probe, the photomultipier delivers a signal at its output, having alevel which depends on the surface potential of the sample, andtherefore can be employed as the secondary signal.

What is claimed is:
 1. A process for rapid spectrum analysis of arecurring signal at at least one measurement point on a sample and formeasuring the spatial distribution of an individual spectral componentof said signal on said sample, comprising the steps of:generating aprimary beam which impinges on the sample and interacts therewith in amanner dependent on said recurring signal, to produce a stream ofsecondary radiation; deriving a secondary signal which is influenced bysaid interaction; performing a frequency conversion on an output from alocal oscillator of a spectrum analyzer to derive a first modulationfrequency; modulating said primary beam with said first modulationfrequency, whereby said secondary signal contains an intermediatefrequency which is a mixture of a frequency of said first modulationsignal and a frequency of said spectral component; and transforming asignal component of said secondary signal at said intermediate frequencyinto an input frequency range of said spectrum analyzer by means of asecond frequency conversion.
 2. A process according to claim 1, whereinsaid input frequency range of said spectrum analyzer is an inputselection range thereof.
 3. A process according to claim 1, wherein saidinput frequency range of said spectrum analyzer is an internalintermediate frequency range of said spectrum analyzer.
 4. A processaccording to claim 1, wherein said spectrum analyzer is operated in asweeping operation.
 5. A process according to claim 2, wherein saidspectrum analyzer is operated in a sweeping operation.
 6. A processaccording to claim 3, wherein said spectrum analyzer is operated in asweeping operation.
 7. A process according to claim 1, wherein, saidspectrum analyzer is operated in a "zero span" mode, the output signalof said spectrum analyzer is transmitted to a recording means, at leastone signal carrying spatial information about the respective measuringpoint is transmitted to said recording means, and said primary beam ispositioned successively at different measuring points.
 8. A processaccording to claim 1, wherein said interaction between said primary beamand said sample comprises the generation of secondary electrons by saidprimary beam and the subsequent influence of the potential contrast onsaid secondary electrons.
 9. A process according to claim 7, whereinsaid interaction between said primary beam and said sample comprises thegeneration of secondary electrons by said primary beam and thesubsequent influence of the potential contrast on said secondaryelectrons.
 10. Apparatus for rapid spectrum analysis of a recurringsignal at at least one point of measuring on a sample and for measuringthe spatial distribution of an individual spectral component of saidsignal on said sample, comprising:a primary beam source for generating aprimary beam; a device for guiding and focussing said primary beam ontosaid sample, which device is supplied with supply voltages andtriggering signals by a triggering means; a device for deriving asecondary signal from said sample, with said secondary signal beinginfluenced by an interaction between said primary beam and the sample; adevice for modulating said primary beam with a modulation frequency; aspectrum analyzer; a first frequency converter, a first input of whichis connected to a local oscillator output of said spectrum analyzer, andan output thereof is connected to said device for modulation; a secondfrequency converter, the first input of which is connected to saiddevice deriving said secondary signal, and an output thereof isconnected to an input of said spectrum analyzer.
 11. Apparatus accordingto claim 10, wherein said primary beam source comprises an electron gunfor generating an unmodulated stream of primary particles and a beammodulation system connected thereto as a device for modulation. 12.Apparatus according to claim 10, wherein a beam modulation generatortransmits a modulation signal as an input to said device for modulating,with an input of said beam modulation generator being connected to theoutput of said first frequency converter.
 13. Apparatus according toclaim 11, wherein a beam modulation generator transmits a modulationsignal as an input to said device for modulating, with an input of saidbeam modulation generator being connected to the output of said firstfrequency converter.
 14. Apparatus according to claim 10, wherein anoutput frequency of said first frequency converter deviates from anabsolute magnitude of a difference between a frequency of the localoscillator output and a frequency of a first internal intermediatefrequency of said spectrum analyzer by a constant frequency value, withsaid constant value being within a frequency range that can be detectedby said device for deriving said secondary signal.
 15. Apparatusaccording to claim 11, wherein an output frequency of said firstfrequency converter deviates from an absolute magnitude of a differencebetween a frequency of the local oscillator output and a frequency of afirst internal intermediate frequency of said spectrum analyzer by aconstant frequency value, with said constant value being within afrequency range that can be detected by said device for deriving saidsecondary signal.
 16. Apparatus according to claim 10, wherein an outputof said second frequency converter is connected to a selection input ofsaid spectrum analyzer, and said second frequency converter decreases afrequency thereof by the value of said modulation frequency. 17.Apparatus according to claim 11, wherein an output of said secondfrequency converter is connected to a selection input of said spectrumanalyzer, and said second frequency converter decreases a frequencythereof by the value of said modulation frequency.
 18. Apparatusaccording to claim 14, wherein said output of said second frequencyconverter is connected to an input to an internal intermediate frequencystage of said spectrum analyzer (SPA) and said second frequencyconverter decreases a frequency thereof by an absolute magnitude of adifference or sum of an intermediate frequency of said internalintermediate frequency stage and said constant frequency value. 19.Apparatus according to claim 10, wherein an output of said spectrumanalyzer is connected to an input of a recording means, with at last afurther signal carrying information about the site of the respectivemeasuring point being transmitted to said recording means.
 20. Apparatusaccording to claim 19, wherein said recording means is a screen and saidspectrum analyzer is swept synchronously with the deflection of thewriting beam of said recording means.